High Performance FET Devices and Methods Thereof

ABSTRACT

Structure and methods of fabrication are disclosed for an enhanced FET devices in which dopant impurities are prevented from diffusing through the gate insulator. The structure comprises a Si:C, or SiGe:C, layer which is sandwiched between the gate insulator and a layer which is doped with impurities in order to provide a preselected workfunction. It is further disclosed how this, and further improvements for FET devices, such as raised source/drain and multifaceted gate on insulator, MODFET on insulator are integrated with strained Si based layer on insulator technology.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 11/065,816 filedFeb. 25, 2005, which application in turn is a divisional of applicationSer. No. 10/427,233 filed May 1, 2003, now U.S. Pat. No. 6,909,186issued Jun. 21, 2005. Both of these applications are incorporated hereinby reference in their entirety.

This application is also related to application Ser. No. 11/067,186filed Feb. 26, 2005, incorporated herein by reference in its entirety.Application Ser. No. 11/067,186 filed Feb. 26, 2005 is a divisionalapplication of application Ser. No. 10/427,233 filed May 1, 2003, nowU.S. Pat. No. 6,909,186 issued: Jun. 21, 2005.

FIELD OF THE INVENTION

The present invention relates to improving FET device performance as thedevice dimensions decrease. More specifically, the invention describessystems and methods to keep dopant impurities from diffusing through thegate insulator, to lower terminal resistance, and to use strained Si,SiGe, or Ge body on an insulator, in particular with multifaceted gateconfiguration MOSFETs and MODFETs.

BACKGROUND OF THE INVENTION

Today's integrated circuits include a vast number of devices. Smallerand faster devices arising from current device scaling are key toenhance performance, but it is also vital to improve or at leastmaintain its reliability as well. However, as MOSFET, (Metal OxideSemiconductor Field-Effect-Transistor, a name with historic connotationsmeaning in general an insulated gate Field-Effect-Transistor [FET]) andin general FET, devices are being scaled down, the technology becomesmore complex and changes in device structures and new fabricationmethods are needed in order to maintain the expected performanceenhancement from one generation of devices to the next. In this regardthe semiconductor that has progressed the farthest is the primarysemiconducting material of microelectronics, silicon (Si), or morebroadly, Si based materials. One such Si based material of importancefor microelectronics is the silicon-germanium (SiGe) alloy.

There is great difficulty in maintaining performance improvements indevices of deeply submicron generations. Several avenues are beingexplored for keeping device performance improvements on track. Amongthese is the use of either tensilely or compressively strained Si as thebasic semiconducting device material having enhanced carrier mobilityfor electrons and holes in comparison to bulk Si transport. Furtherimprovements can be achieved by alloying Si with Ge. Additionally, afurther commonly used scheme is to build devices in a semiconductinglayer which is isolated from the semiconducting substrate by a buriedinsulating layer. Most commonly the semiconducting layer is Si, hencethe terminology SOI (Si on insulator) is generally in use, and theburied insulator is SiO₂, to yield the name of BOX (buried oxide).However, there are still many outstanding issues in achieving thehighest possible performance in deeply submicron MOSFET devices.

As the gate insulator is thinned, as dictated by the requirements ofever smaller devices, there is the problem of the doping impuritiespenetrating the gate insulator, typically an SiO₂ layer. For the sake ofoptimal device design, the gate typically is made of polysilicon, whichis doped the same conductivity type as the device itself. With suchdoping the resultant workfunction of the gate with respect to thechannel region of the device allows for the threshold of the device tobe optimally set. Accordingly, N-type devices are in need of N-dopedgates, and P-type devices are in need of P-doped gates. During the hightemperatures of device manufacturing, the gate-doping species, mostproblematically boron, (B), but others like phosphorus (P) as well,readily penetrate the thin gate insulator and destroys the device. Thegate insulator in modern high performance devices typically needs to beless than about 3 nm thick. Preventing this dopant penetration would bean important step in achieving thinner gate insulators.

In this invention when the strained monocrystalline layer which ishosting the critical parts of the devices, such as the channel regions,is referred to as a SiGe layer it is understood that an essentially pureSi or Ge layer is included in this terminology.

For high device performance the resistance of a turned on device must beas low as possible. With smaller devices the intrinsic resistance of thedevice itself is decreasing, but other, so called parasitic, resistanceshave to be taken care of. One such resistance arises from the sourceterminal of the device. To minimize both the source and drainresistance, these device regions are typically implanted and thensilicided during device fabrication. However, the consumption of toomuch Si during the silicidation process has, and does create drawbacksof its own. In SOI technologies, where the device is purposely built ina thin device layer over an insulator, this problem is especially acute.The silicide formation can easily consume the whole portion of the thindevice layer in the source and drain regions. Therefore, there is a needfor making the semiconductor device layer thicker especially in thesource and drain regions, or find other means to reduce the effect ofthe source resistance.

With shortening gate lengths the so called short channel effects, mostnotably the “drain induced barrier lowering” (DIBL) pose severeroadblocks to miniaturization. These effects can be mitigated byintroducing basic structural changes in the devices, leading to the useof multiple gates. However, this approach can only yield the desiredperformance improvements if it is appropriately coupled with other highperformance techniques, a problem still looking for solutions.

SUMMARY OF THE INVENTION

In accordance with the objective to achieve high performance downscaledMOSFET devices, the present invention describes a system and method forsolving associated problems and/or in their combinations thereof.Deposition and/or epitaxial growth of appropriate layers, bothcrystalline, and polycrystalline is at the core of most fabricationtechniques that lead to high device performance. The preferred proceduredeposition and growth is ultra high vacuum chemical vapor deposition(UHV-CVD).

It is well known that carbon (C) can serves as a retardant of dopantdiffusion in both Si and SiGe materials and devices. For example, H. J.Osten et. al, in the paper entitled “Carbon Doped SiGe HeterojunctionBipolar Transistors for High Frequency Applications, IEEE BCTM 7.1,1999, pp 109-116, which is incorporated herein by reference, have shownthat low carbon concentrations (<10²⁰ atom/cm³) can significantlysuppress boron out-diffusion without affecting the strain or bandalignment of carbon-rich SiGe:B layers in order to achieve highperformance SiGe heterojunction bipolar transistors. Similarly, Ruckeret. al., in a paper entitled “Dopant Diffusion in C-doped Si and SiGe:Physical Model and Experimental Verification”, IEDM, 1999, pp 345-348,which is incorporated herein by reference, have further shown thatcarbon doping can be used to suppress P diffusion as well, in additionto suppressing the transient enhanced diffusion (TED) behavior of B.With appropriate techniques, such as UHV-CVD, during Si deposition C canbe distinctly incorporated to a few percent into crystalline, orpolycrystalline Si films without any residual oxygen contamination oftenassociated with other carbon doping or growth techniques. The resultingmaterial Si:C is practically a stopping layer for diffusing electricallyactive impurities, such as boron or phosphorous. The technique can alsobe used when C is incorporated into SiGe during deposition, givingSiGe:C films. With UHV-CVD one can deposit ultra-thin, device qualitylayers of Si:C or SiGe:C up to approximately 10% of C content. Disposingsuch a dopant barrier layer onto the gate insulator, prior to thedeposition of the doped layer, has the desired effect of protecting thechannel region underneath the gate insulator without upsetting theelectrical properties of the device. Since from etching behavior, or apatterning point of view the properties of Si:C and SiGe:C are almostthe same as those of pure Si, the Si:C or SiGe:C layer would not needany special processing, such as additional patterning, or etching stepbeyond those typical of standard MOSFET fabrication. The Si:C or SiGe:Ccan be deposited in blanket, or borderless, manner just prior to thedeposition of the bulk of the gate material, which is typically dopedpolysilicon. It is the objective of this invention to teach theprevention of dopant impurity penetration of the gate insulator bydepositing a Si:C or SiGe:C layer directly over the gate insulator toserve as the dopant diffusion barrier.

Increasing the thickness of the semiconductor over the source andregions prior to silicidation is very desirable, since state of the artdevices are shallow structures with shallow source/drain junctions, andthere is danger for the silicide to punch through the junctions.Selective epitaxial growth is a well developed technique especially inRT-CVD, but it is also possible to achieve in UHV-CVD fabrication ofstrained Si and SiGe layers. Such selectivity is of great use indepositing extra material over the desired areas. In the many steps oftypical MOSFET device fabrication there is a point where the gate andthe source/drain regions undergo a so called self-aligned silicidationprocess. This means that although there may be many different opensurfaces on the wafers there is no need for masking, the silicidationprocess itself is executed in a manner that it selects the desiredregions where it deposits, mostly over exposed semiconductor surfaces.Moreover, the selective CVD process can also be tuned to deposit onlyover the source, the source and the drain, or additionally over the gateand other desired areas, such as contacts and polysilicon interconnects.With such a selective deposition, just prior to the deposition of thesiliciding metal, such as Ni, Co, Ir, Ti, W, and Pt the semiconductormaterial is thickened in precisely the desired regions of the sourceand/or the drain. The following silicidation step will now be able toavail itself to more semiconductor material for complete consumption andfinal formation of the metal silicide. It is the objective of thisinvention to combine the technique of raising the source/drain junctionsby selective deposition, with the technique of disposing Si:C, orSiGe:C, over the gate insulator, and with the technique of using highperformance strained SiGe over insulator device materials.

Along the path of seeking ever higher device performances, downscalingof MOSFET devices is the established guiding principle for current CMOSdevice technology. However, there are visible limits to straightforwarddownsizing as short-channel effects (SCE) becomes a major problem andconcern when devices are scaled down to the nanometer regime. A proposedway out of this problem is the use of multifaceted gate devices. Such adevice is not simply a planar structure conducting on one surface, butconducting on more than one side, or facet on the surface of a devicebody. The reasons that a multifaceted gate device can be downscaledfurther than a regular planar device are relatively complex, but theyhave been already given in the technical literature, for instance in:“Device Design Considerations for Double-Gate, Ground-Plane, andSingle-Gated Ultra-Thin SOI MOSFET's at the 25 nm Channel LengthGeneration,” by H.-S. P. Wong, et al, 1998 IEDM Tech Dig., pp. 407-10.It is a further object of the present invention to teach highperformance ultra short devices obtainable with the combination ofstrained Si or SiGe on insulator technology, multifaceted gatetechniques, and with the ultra smooth channel interfaces allowed byepitaxial gate oxide deposition.

MODFET (Modulation Doped FET) devices offer another avenue toward highperformance. MODFET devices as such are known in the art. However, thesame techniques, such as wafer transfer, used in creating multifacetedgate MOSFETs can be used to create novel MODFET devices on insulator.These novel MODFETs are hosted in a strained Si based layer directly onthe insulator, without any intervening conducting, or semiconductingbuffer layer. The term of hosting a device in a certain material, orlayer, means that the critical part of the device, that which is mainlysensitive to carrier properties, such as, for instance, the channel ofMOS, or MODFET devices, is residing in, composed of, housed in, thatcertain material, or layer.

There is a large number of patents and publication relating to thesubject of high performance MOSFET devices. They cover some aspects ofimproving MOSFET performance, but none teaches the full extent of thepresent invention. The references that follow, give background materialsfor the present invention.

U.S. Pat. No. 6,524,935 entitled: “Preparation of Strained Si/SiGe onInsulator by Hydrogen Induced Layer Transfer Technique” to D. Canaperiet al, incorporated herein by reference, teaches strain layer depositionand Hydrogen induced layer transfer (SmartCut), but it does not teachthe present invention.

Formation of SiGe layers can proceed as described in U.S. Pat. No.5,659,187 to LeGoues et al. titled: “Low Defect Density/arbitraryLattice Constant Heteroepitaxial Layers” incorporated herein byreference.

Fabrication of a tensilely strained SiGe layer, and use of C inconjunction with Si and SiGe is taught in US patent application titled:“Strained Si based layer made by UHV-CVD, and Devices Therein”, by J. O.Chu et al, filed Feb. 11, 2002, Ser. No. 10/073,562, (Now U.S. Pat. No.6,649,492) incorporated herein by reference, but this application doesnot teach the present invention.

Fabrication of both tensilely and compressively strained SiGe layers onthe same insulator, and how to achieve ultra thin strained layers oninsulator is taught in US patent application titled: “Dual Strain-StateSiGe Layers for Microelectronics”, by J. Chu, filed Mar. 15, 2003, Ser.No. 10/389,145 (Now U.S. Pat. No. 6,963,078), incorporated herein byreference, but this application does not teach the present invention.

Formation of raised source/drain is described in U.S. Pat. No. 6,063,676to Choi et al, titled: “Mosfet with raised source and drain regions” butthis patent does not teach the present invention.

Fabrication of a double gate MOSFET on SOI is taught in U.S. Pat. No.6,352,872 to Kim et al, titled: “SOI device with double gate and methodfor fabricating the same”, but this patent does not teach the presentinvention.

In preferred embodiments of the invention, the fabrication steps leadingto the described device improvements are done by UHV-CVD processes, andpreferably in an AICVD system as described in U.S. Pat. No. 6,013,134 toJ. Chu et al, titled: “Advance Integrated Chemical Vapor Deposition(AICVD) for Semiconductor Devices”, incorporated herein by reference.

MODFET devices have been previously built in SiGe layers where thecomposition of the layers was tailored for device properties. Such isthe invention of U.S. Pat. No. 5,534,713 to K. Ismail and F. Stern,titled “Complementary metal-oxide semiconductor transistor logic usingstrained SI/SIGE heterostructure layers” incorporated herein byreference, where the details of the MODFET structure and fabricationthereof can be found. However, this patent does not teach the presentinvention, where the layer hosting the device is directly on theinsulator.

The invention further teaches the devices hosted in the strained Sibased layers on insulators, which can operate from 400° K to 5° K, andteaches processors functioning with such devices. The high end of theapproximate range, 400° K, although achievable with the high performancetechniques disclosed herein, it is not the most preferable for theoptimal FET performance. High performance is associated with straineddevice layers, and SOI technology, and also with low temperatureoperation. Device performance (for FET type devices) improves withdecrease in temperature. To obtain the optimal performance of devices atlow temperatures they have to be device-designed already for lowtemperature operation. Such device-designs, optimized for lowtemperature operation, are well known in the previous art. A desirabletemperature range for low temperature high performance FET operation isbetween about 250° K and 70° K. This invention, by combining thedevice-designs for operation in the 400° K to 5° K range with the SOItechnology and with the both tensilely and compressively strained devicelayers directly on the insulators aims at devices and processors of theutmost performance. Devices where the strained layers are directly onthe insulators are especially suitable for low temperature operation dueto their low capacitance. Also, multifaceted gate devices haverelatively large surfaces which helps decreasing source/drainresistance, another advantage for low temperature operation. An allcases where Shottky junction electrodes are utilized, as in several ofthe high performance devices disclosed herein, benefit from lowtemperature operation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will become apparentfrom the accompanying detailed description and drawings, wherein:

FIG. 1 Shows a cross sectional view of a layered structure to be used ina MOSFET device for preventing dopant penetration across the gateinsulator;

FIG. 2 Shows a schematic cross sectional view of a MOSFET deviceincorporating the layered structure for preventing dopant penetrationacross the gate insulator;

FIG. 3 Shows a schematic cross sectional view of a MOSFET deviceincorporating raised source/drain regions and the layered structure forpreventing dopant penetration across the gate insulator;

FIG. 4 Shows a schematic cross sectional view of a MOSFET as in FIG. 3after a silicidation step has been completed;

FIG. 5 Shows schematically embodiments of a strained Si basedmonocrystalline MOSFET on insulator with multifaceted gate, with currentflow in parallel with the plane of the supporting platform;

FIG. 6 Shows schematically, in a side view and in a cross sectionalview, another embodiment of a strained Si based monocrystalline MOSFETon insulator with multifaceted gate, with current flow in parallel withthe plane of the supporting platform;

FIG. 7 Shows schematically, in a side view and in a cross sectionalview, a further embodiment of a strained Si based monocrystalline MOSFETon insulator with multifaceted gate, with current flow in parallel withthe plane of the supporting platform;

FIG. 8 Shows schematically an embodiment of a strained Si basedmonocrystalline MOSFET on insulator with multifaceted gate, with currentflow in perpendicular to the plane of the supporting platform;

FIG. 9 Shows schematically an alternate embodiment of a strained Sibased monocrystalline MOSFET on insulator with multifaceted gate, withcurrent flow in perpendicular to the plane of the supporting platform;

FIG. 10 Shows schematically a MODFET device hosted in a strained Sibased layer directly on an insulator; and

FIG. 11 Shows schematically an electronic system comprising of strainedSi based monocrystalline strip multifaceted gate MOSFET on insulatordevices.

DETAILED DESCRIPTION OF THE INVENTION

As the gate insulator, usually SiO₂, thickness is being scaled down,i.e. below 5-10 nm, there is strong tendency that during any annealing,or rapid thermal annealing (RTA) steps, which are part of the devicemanufacturing process, the dopant originally in the polysilicon layer onthe gate insulator diffuses through the gate insulator into the channelregion of the MOSFET body. These dopants in the channel region woulddramatically degrade the performance of the device. The doping level inthe gate for 100 nm devices has to be at a very high level, i.e.>10²¹/cm³. The common P-type dopant Boron (B) is the most prone topenetrate the gate insulator.

It was experimentally found that when a gate stack was made with twolayers, a thin Si:C over the SiO₂, followed by a heavily B dopedpolysilicon layer, then after annealing the gate using RTA at 1000° C.for 60 seconds, much of the boron from the P⁺⁺ polysilicon layer was“arrested” within the poly-Si:C barrier layer, and very little dopantdiffused through the oxide. On the other hand, when the poly-Si:C layerwas not present, the boron dopants from the P⁺⁺ polysilicon layer havepenetrated through the gate oxide as expected. The ability to growdevice quality poly-Si:C or poly-SiGe:C films is the result of a newlydeveloped UHV-CVD carbon process using ethylene as the preferred carbonprecursor where no residual oxygen contaminations are present, orincorporated, during the carbon growth process. The growth of devicequality poly-Si:C or poly-SiGe:C by UHV-CVD is further described in USpatent application titled: “Epitaxial and Polycrystalline Growth ofSi_(1-x-y)Ge_(x)C_(y) and Si_(1-y)C_(y) Alloy Layers on Si by UHV-CVD,by J. O. Chu filed Apr. 20, 2001 (Now U.S. Pat. No. 6,750,119),incorporated herein by reference. Now it is possible to grow theP++poly-silicon and the poly-Si:C layers over an oxide layer in aborderless fashion. Moreover, adding this thin poly-Si:C or poly-SiGe:Cbarrier layer will not have a significant effect on the resistivity orthe electrical properties of the gate electrode. In the embodiment whenemploying a poly-SiGe:C barrier layer the overall resistivity of thegate electrode can improve due to the presence of germanium in the gatestack.

FIG. 1 shows a cross sectional view of a layered structure, to be usedin a MOSFET device for preventing dopant penetration across the gateinsulator. Layer 160 serves for hosting the body of the MOSFET device.This body 160 can be part of a Si substrate, or part of a device layeron top of an insulator, as in SOI technology. For highest performancedevices the body layer 160 would be a strained Si, SiGe including almostpure Ge, having enhanced carrier mobility, layer over an insulatorstructure. Over the channel region of the body is a gate insulator 150.This insulator usually is SiO₂, but it could be Al₂O₃, HfO₂, or Ta₂O₅,or any other gate insulator. On top of the insulator 150 one deposits,usually by UHV-CVD, an ultra-thin, device quality, poly Si:C or polySiGe:C 100 with a C content in the range from 0.5% to 10%. Disposingsuch a first layer, of Si:C or SiGe:C 100, with a thickness about 1 nmto 5 nm, onto a gate insulator 150 prior to the deposition of the secondlayer (the doped layer 110), has the desired effect of protecting thechannel region underneath the gate insulator without upsetting theelectrical properties of the device. Since the material properties ofSi:C and SiGe:C from an etching or a patterning point of view arepractically identical to those of pure Si, the Si:C or SiGe:C layerwould not need any additional patterning, or etching step beyond thosetypical of standard MOSFET fabrication. The initial Si:C or SiGe:C ispreferred to be deposited in a blanket, or borderless, manner just priorto the deposition of the bulk of the gate material 110, which istypically polysilicon. The gate insulator penetration by a dopant is themost problematic for the case when the dopant is B, however barrier theeffect of the Si:C and SiGe:C is not limited to B alone. Layer 150offers protection against the diffusion of other P-type dopants andagainst N-type dopants, such as P, as well.

FIG. 2 shows a schematic cross sectional view of a MOSFET deviceincorporating the layered structure for preventing dopant penetrationacross the gate insulator. Here again, the device region of the MOSFETis shown by body layer 160, but now it further includes the fabricatedsource/drain regions 260 underneath the gate stack in FIG. 2A, andShottcky-barrier contact source/drain 260 in FIG. 2B. FIG. 2A as shownfurther describes the case when the device is a P-MOS with P-type sourceand drain, but this should not be read as a limitation. It is importanthowever, that the type of device, namely whether it is a P or an N type,determines what kind of impurities have to be in the second layer, thedoped gate layer 110. The gate has to be doped with the properimpurities to provide a selected, or desired, workfunction with respectto the channel region of the device. This workfunction determines to alarge extent the threshold voltage of the device. Layer 150 is the gateinsulator, and 100 is the first layer, the Si:C or SiGe:C diffusionpreventing layer. In FIG. 2 these layers are shown as already havingformed into a gate stack over the body layer 160.

In some embodiments, typically when the gate is shorter than about 50nm, there maybe an advantage in using a Schottky-barrier contact for thesource-to-channel interface. The term “Schottky-barrier contact” isstandard nomenclature for a contact between a semiconductor and a metal.Accordingly, as shown for a representative embodiment in FIG. 2B, thesource, or source/drain 260 silicidation process can be allowed toproceed until the silicide 430 meets the channel region under, or at theedge of the gate stack, and indeed for it to consume all of the sourcejunction. In the process, the drain junction might, or might not, turninto a Schottky-barrier contact, but either way is acceptable, since thedrain junction resistance has much less importance for deviceperformance than the source junction resistance.

FIG. 3 shows a schematic cross sectional view of a high performanceMOSFET device incorporating raised source/drain regions and the layeredstructure for preventing dopant penetration across the gate insulator.An insulator layer 370, typically a BOX, supports and electricallyisolates the other layers of the device. Layers 350 and 360 on top ofthe BOX layer 370 form the thin device layer. Layer 350 on the top isthe one which hosts the channel region under the gate insulator 150.Layer 350 is a strained, either tensilely or compressively, SiGe layer,typically between about 2 nm and 50 nm thick. Layer 360 is a supportinglayer, typically a SiGe relaxed buffer. The part of the gate stack shownis same as in FIG. 2: the diffusion preventing layer 100, and the dopedlayer 110, typically polysilicon. The source and drain regions of thedevice layer 310, have been raised by depositing layer 300. This isaccomplished by selective deposition of Si, Si:C, SiGe, or SiGe:C ontoexposed semiconductor regions. FIG. 3 is only schematic, it does notshow features that are not central to the embodiment, such as possibleside-walls or spacer layers on the gate, and many other details. Withthe additional semiconducting material 300 over the junctions 310, onedecreases the chances that upon forming a silicided contact, thesilicide consumes too much of the junction regions, which would usuallybe detrimental for device behavior.

FIG. 4 shows a schematic cross sectional view of a high performanceMOSFET as the one in FIG. 3, after a silicidation step has beencompleted. The silicided regions 430 form contacts over the source anddrain, and forms a metallic layer over the gate for improved gateconductivity. Because of the additional semiconducting materialavailable in the raised regions 300′, the source/drain silicide processallows the source/drain contacts to be completely silicided andconsequently to be fully electrically functional. The notation 300′indicates that this is the region which was occupied by the raised partof the source/drain, however, after the silicidation there isessentially only a uniform block of silicide 430. The metals which aretypically used for forming silicide with silicon are Ni, Co, Pt, Ti, W,Ir, or any of their combinations thereof.

The effect of source terminal resistance is generally more important indevice behavior that the effect of drain terminal resistance. Oneskilled in the art would notice, that the described selective epitaxyfor raising a terminal can be employed for each terminal independentlyof the other terminal. In other words, the source and the drain can beraised individually, both at the same time, or both in sequence.

FIG. 5 shows schematically, embodiments of a strained Si basedmonocrystalline MOSFET on insulator, with a multifaceted gate, having acurrent flow in parallel with the surface plane of the supportingplatform. FIG. 5A and FIG. 5B show two views of a strained SiGe MOSFETon insulator where the gate 500 comprises two electrodes on the bottomfacet and a top facet of the channel region of the Si based strainedstrip. The strained Si based monocrystalline strip typically is Si,SiGe, Si:C, SiGe:C, or approaching almost pure Ge. Direction of devicecurrents in this device, shown as a thick arrow 501, is in paralleldirection with the surface plane 596 of the supporting platform 595,590, and bottom electrode 500, wherein the supporting platform has asurface plane 596, that plane which is interfacing with the Si basedstrained strip, or layer segment. This indicates that the device is in aso called “horizontal configuration”. FIG. 5A shows the side view of thedevice, and FIG. 5B is a cross sectional view along the brokencenterline “a” of FIG. 5A. In this embodiment a strained Si basedmonocrystalline strip 510 has channel regions on two of its facets, orsides. One is on the bottom facet 511 which is the one bonding to saidsupporting platform, and another channel region is on at least one ofthe top facets 512, with the side facets not taking part in deviceaction. The whole device rests on a substrate 590, typically Si, with aninsulator layer on top 595. The device is double gated, the gate 500 hastwo electrodes on two facets of the multifaceted strained body 510,overlaying the channel regions and 511 and 512 interfacing with the gateinsulator. In this device layers 595, 590, and the gate electrode 500,engaging the insulator 595 together are forming the supporting platform.

The gate insulator comprises an epitaxial SiO₂ layer 520 which is grownonto the strained body, interfacing the channel region and it serves toprovide the highest quality interface between the gate insulator and thestrained Si based monocrystalline strip. The epitaxial SiO₂ layer istypically less than 2 nm thick, and normally it is covered by anadditional insulating layer 530, which in the most part is non-epitaxialor amorphous SiO₂. In the figure, layers 520 and 530 together comprisethe gate insulator, however one skilled in the art would recognize, thatlayer 530 itself may be a composite layered structure, or in some othersituations layer 530 can be completely omitted. Looking at FIG. 5A, theregion of the strip 510 between the two thick broken lines is the onebetween the two gates. Those sides of the device which are not under thegate influence, would typically be covered by a passivating insulator,such as SiO₂ or even by an air gap, to render them electrically neutral.The figure does not show this passivating insulator, since passivatinginsulators are well known in the art. Beyond the gate controlled regionof the body, the strip is to be made into a source and a drain 540respectively. Methods for source/drain formation are known in the art.To assure low source/drain resistance the regions 540 are typicallyimplanted and then silicided afterwards. In FIG. 5B the same structurehas been rotated by 90° showing in a cross sectional view along thebroken centerline “a”. The direction of device currents 501 is nowperpendicular to the plane of the drawing, and the arrow 501 indicatingthe direction of device current flows, looking from a head onperspective and is depicted as concentric circles. Of course the devicecurrents are in parallel direction with the surface plane of thesupporting platform 596.

FIG. 5C and FIG. 5D show two views of a strained SiGe MOSFET oninsulator where the gate 500 comprises two electrodes on the side facetschannel region of the Si based strained strip. Direction of devicecurrents in this device, shown as a thick arrow 501, is in parallel withthe surface plane 596 of the supporting platform 595, 590. FIG. 5D showsat least two opposing side facets 513, and the two separate gateelectrodes 500 that are engaging the two opposing side facets 513.Direction of device currents are indicated by arrow 501, which are inparallel with the surface plane 596 of the supporting platform 595 and590. This device is considered to be in a so called “horizontalconfiguration”. The multifaceted device configuration schematicallyshown in FIGS. 5C and 5D is also sometimes referred in the art as FinFETdevice configuration.

Details of the fabrication of the strained Si or SiGe strip 510, or ingeneral of a strained Si based material layer such as 570 in FIG. 10,and the way the strip is engaged by bonding means to the supportingplatform can be found in the earlier incorporated references:application filed by J. Chu et al on Feb. 11, 2002, Ser. No. 10/073,562,(Now U.S. Pat. No. 6,649,492), and application filed by J. Chu on Mar.15, 2003, Ser. No. 10/389,145, (Now U.S. Pat. No. 6,963,078). Briefly,the strained Si, SiGe, Si:C, SiGe:C, or Ge layer is grown over a firstsubstrate and a support structure, and then transferred to thesupporting platform. The supporting platform is a second substrate 590,the insulator 595, and in some embodiments that part of a the gate 500which rests on insulator 595. The support structure is removed from thestrained Si or SiGe layer by use of selective etching. A thin, pure Si,or pure Ge, layer abutting the strained Si or SiGe layer plays a centralrole in stopping the etching once the support structure is consumed bythe etchent. An epitaxial oxide layer on top of the strained Si, orSiGe, layer, grown before the layer transfer, can promote adhesion tothe new supporting platform, and also helps in preserving the strainstate of the strained Si, or SiGe, layer. This epitaxial oxide layerwill in some embodiments also turned into a part of the gate insulator520. An additional insulator on top of the epitaxial oxide layer canalso be applied, which then will be turned into that portion of layer530 which faces the supporting platform. In some embodiments of themultifaceted devices, the receiving substrate of the layer transfer, thesecond substrate, the one which is part of the supporting platform, isprepared with a polysilicon, or a metal, typically silicide, or acombination of the two, on its top, and this polysilicon/metal layerwill become part of the multifaceted gate. For the embodiment of FIGS.5A and 5B this polysilicon/metallic layer is to become the bottom gateelectrode of the gate 500. There can be embodiments where the bondingmeans for strained Si, or SiGe, strip, or layer, to the supportingplatform does not involve an epitaxial oxide, or polycrystalline Si, orsilicide, and it is simply a bonded interface formed during the a layertransfer step between the insulator, typically SiO₂, and the strained Sibase material strip, or layer.

Once the layer transfer and the removal of the support structure byetching has been done, and one has the strained Si based material, layereither on the supporting platform or directly on the insulator layer,the fabrication of the strained body strips with its desiredmultifaceted gate configuration can readily be achieved based uponprocedures which are well known in the art of silicon CMOS devicefabrication and integration. In contrast, while various combinations ofmasking, patterning, etching by wet etch, etching by reactive ionetching (RIE), or many similar steps that are used to create the finaldevice structures are well known in the art, novel steps, like thedeposition of Si:C, or SiGe:C, for diffusion barriers as part of thegate preparation are part of this invention.

FIG. 6 shows schematically, in a side view and in a cross sectionalview, another embodiment of a strained Si based monocrystalline MOSFETon insulator, with a multifaceted gate, with current flow in parallelwith the plane of the supporting platform. FIG. 6A shows the side viewof the device, and FIG. 6B is a cross sectional view along the brokencenterline “a” of FIG. 6A. The embodiment depicted in FIG. 6 differsfrom that in FIG. 5 only in that the gate now surrounds the strained Sibased body completely. It forms a sort of belt around the body.Accordingly, in the side view of FIG. 6A the body 510 is not visible.From the side, only the gate 500 and the source/drain regions 540 arevisible. Direction of device currents are indicated by arrow 501, whichare in parallel with the surface plane 596 of the supporting platform595 and 590. All aspects, and fabrication considerations are the same asdescribed relating to the embodiment shown in FIG. 5. The device of FIG.6 is also in a “horizontal configuration”.

FIG. 7 shows schematically, in a side view and in a cross sectionalview, a further embodiment of strained Si based monocrystalline MOSFETon insulator, with a multifaceted gate, and with current flow inparallel with the plane of the supporting platform. FIG. 7A shows theside view of the device, and FIG. 7B is a cross sectional view along thebroken centerline “a” of FIG. 7A. The embodiment depicted in FIG. 7differs from that in FIG. 6 in that here the gate engages all of thefacets of the strip with the exception of one facet: that facet, whichbonds to the supporting platform. In this embodiment the supportingplatform does not include a polysilicon or metallic layer. Thesupporting platform in FIG. 7 includes only the substrate 590 andinsulator layer 595. FIG. 7 does not show a thin epitaxial oxide on thebottom facet of the body 510, which is the bonding facet the supportingplatform. In this embodiment the strained Si, Si:C, SiGe, SiGe:C, almostpure Ge, or almost pure Ge:C strip may, or may not, include such anepitaxial oxide. Such an oxide is desirable to promote adhesion to thenew supporting platform, and can also help in preserving the strainstate in the strained Si or SiGe layer. However, since in thisembodiment there is no gate electrode on the bottom facet, a gateinsulator and hence epitaxial oxide, are not a necessity. Direction ofdevice currents are indicated by arrow 501, which are in parallel withthe surface plane 596 of the supporting platform 595 and 590. Allaspects, and fabrication considerations are the same as describedrelating to the embodiment shown in FIG. 5. The device of FIG. 7 is alsoin a “horizontal configuration”.

FIG. 8 shows schematically, an embodiment of a strained Si basedmonocrystalline MOSFET on insulator with multifaceted gate, with currentflow in perpendicular to the plane of the supporting platform. Thisembodiment has a gate 500 completely surrounding the multifacetedchannel region (invisible due the gate.) Direction of device currents inthis device, shown as a thick arrow 501, is in perpendicular directionto the surface plane 596 of the supporting platform 595, 590. Thisindicates that the device is in a so called “vertical configuration”.The embodiment of this strained SiGe MOSFET on insulator withmultifaceted gate of FIG. 8, apart of its orientation, in other aspectsand its fabrication it is practically identical with the embodimentdepicted for FIG. 6.

FIG. 9 shows schematically, an alternate embodiment of a strained Sibased monocrystalline MOSFET on insulator with multifaceted gate, withcurrent flow in perpendicular to the plane of the supporting platform.The device is double gated, where the gate 500 has two electrodes on twoside facets 513 of the multifaceted strained body 510, overlaying thechannel regions and interfacing with the gate insulator 520 and 530.Direction of device currents in this device, shown as a thick arrow 501,is in perpendicular direction to the surface plane 596 of the supportingplatform 595, 590. This indicates that the device is in a so called“vertical configuration”. The embodiment of this strained SiGe MOSFET oninsulator with multifaceted gate of FIG. 9, apart of its orientation, inother aspects and its fabrication it is practically identical with theembodiment depicted on FIG. 5C and FIG. 5D. The multifaceted deviceconfiguration schematically shown in FIG. 9 can be referred to as a“vertical FinFET” device configuration.

To increase current carrying capacity, for the multifaceted gatestructures in general, a multiple-fingered gate configuration can beemployed.

For all of these embodiments, as shown in FIGS. 5, 6, 7, 8, and 9 asub-embodiment in the source drain formation is possible. There areindications that when device channels are truly short, belowapproximately 50 nm, there maybe an advantage in using aSchottky-barrier contact for the source to channel junction. A processfor fabricating CMOS devices with self-aligned Schottky source and drainhas been described in a paper entitled: “New Complimentary Metal-OxideSemiconductor Technology with Self-Aligned Schottky Source/Drain andLow-Resistance T Gates” by S. A. Rishton, et al, J. Vac. Sci. Tech. B 15(6), 1997, pp. 2795-2798 and is incorporated herein by reference.Accordingly, in all these multifaceted devices, or planar devices asshown on FIG. 2, the source, or source/drain silicidation process can beallowed to proceed until the silicide meets the channel region, wherebyit has indeed consumed all of the source/drain junctions. In the processthe drain junction might, or might not, turn into a Schottky-barriercontact, but either way is acceptable, since the drain junctionresistance is not as important for device performance as the sourcejunction resistance is. Similarly, a selective or a sequential two-stepsource/drain silicidation process could be employed to create thedesired Schottky-barrier contact only for the source region whilekeeping the normal (low resistance) silicidation process for the drainregion the same.

FIG. 10 shows schematically a MODFET device 601 hosted in a strained Sibased layer directly on an insulator. FIG. 10 does not go into detail ofthe MODFET device 601, since such a device is well known in the art.Fabrication of MODFETs in the specific material environment of the Sibased strained materials is given for instance in the alreadyincorporated references of U.S. Pat. No. 5,534,713 to K. Ismail and F.Stern, titled “Complementary metal-oxide semiconductor transistor logicusing strained SI/SIGE heterostructure layers”, and application filed byJ. Chu on Mar. 15, 2003, Ser. No. 10/389,145, (Now U.S. Pat. No.6,963,078).

The MODFET device 601, independently whether it is an N-MODFET, orP-MODFET, is hosted in the Si based strained layer 570. It is importantthat the Si based strained layer 570 is directly on the insulator layer595 without any intervening conducting, or semiconducting buffer layer.Such an arrangement allows for unprecedently low device capacitancesresulting in superior high speed device performances. The layer 570depending of the need of the device can be either tensilely orcompressively strained. The critical part of the device, such as thechannel 610, is hosted in strained layer 570. This strained Si basedlayer, typically Si, SiGe, or SiGe:C, or possibly close to pure Ge orGe:C is directly on an insulator 595. The insulator layer 595, typicallySiO₂, is engaging the strained Si based monocrystalline layer by bondingmeans. These means are the same as given in relation to the multifacetedgate devices on insulator, described on FIGS. 5 to 9. The insulatorlayer 595 is on top a substrate 590, typically a Si wafer. In thisembodiment layers 595 and 590 together form the supporting platform.Other usual parts of the MODFET device 601, such as source and drain 540and auxiliary layers 620 are fabricated by means known in thesemiconducting manufacturing arts.

FIG. 11 shows schematically an electronic system comprising ofmultifaceted gate strained Si based monocrystalline strip MOSFET oninsulator devices. The electronic system 900 can be any processor whichcan benefit from the high performance afforded by the strained SiGeMOSFET on insulator with multifaceted gate devices. These devices formpart of the electronic system in their multitude on one or more chips901. Embodiments of electronic systems manufactured with the strainedSiGe MOSFET on insulator with multifaceted gate devices are digitalprocessors, typically found in the central processing complex ofcomputers; mixed digital/analog processors, which benefit significantlyfrom the high mobility of the carriers in the strained SiGe; and ingeneral any communication processor, such as modules connecting memoriesto processors, routers, radar systems, high performance video-telephony,game modules, and others.

Many modifications and variations of the present invention are possiblein light of the above teachings, and could be apparent for those skilledin the art. The scope of the invention is defined by the appendedclaims.

1. A method for fabricating a MOSFET device, comprising: hosting achannel region for said MOSFET device in a Si based crystalline body;providing a gate insulator layer disposed on top of said body;depositing onto said gate insulator layer a first layer, wherein saidfirst layer is Si:C; and disposing a second layer on top of said firstlayer, wherein said second layer is comprising polycrystalline Si, andwherein said second layer is doped with impurities to provide a selectedworkfunction with respect to said channel region.
 2. The method of claim1, wherein said first layer is deposited with about 0.5% to 10% of Cconcentration.
 3. The method of claim 1, wherein said first layer isdeposited to a thickness of about 1 nm to 5 nm.
 4. The method of claim1, wherein the step of depositing said first layer is executed as aborderless deposition.